Septic treatment systems are used to treat and dispose of effluent, or wastewater, from residences and commercial buildings. Septic treatment systems are onsite systems that do not involve central wastewater collection and treatment as in municipal water treatment. A typical septic treatment system includes a septic tank and a leach field, or drain field, and the entire system is located underground. The septic tank is a conduit that essentially digests organic matter and separates floatable matter, such as oils and grease, and non-floatable solids from the effluent. Effluent is then discharged from the septic tank into a distribution pipe that drains into a leach field. The leach field contains a leach-field system that either is composed of gravel or is gravel-less. The leach-field system is designed to slowly release effluent into the soil, which purifies the effluent.
A leach field serves two key functions: to dispose of effluent from septic tanks and to distribute effluent in a manner allowing adequate, natural wastewater treatment in the soil before any harmful contaminants reach the underlying groundwater aquifer. Leach-field systems are integral in removing or neutralizing pollutants like disease-causing pathogens, nitrogen, phosphorus, and other contaminants. When any part of the septic system is inappropriate or inadequate based on the conditions or other factors, failures occur, including contamination of underground water aquifers. The most serious documented problems involve contamination of surface waters and ground water with disease-causing pathogens and nitrates. When working properly, a leach-field system allows the surrounding soil to filter the pollutants in the effluent before the effluent reaches the underlying groundwater aquifer. The soil reduces or completely removes harmful contaminants from the effluent.
Leach fields, or drain fields, are typically classified as either gravel or gravel-less systems. Historically, gravel systems (e.g., gavel and pipe systems) have been used in leach fields. In gravel systems, perforated pipes that distribute the effluent are placed over a layer of gravel underground. Then four inches of additional rock are placed around the pipe and two inches above the pipe. The installation of a gravel system creates a problem in that the drain field could be compromised by the compaction of moist soil resulting from the use of heavy equipment that is necessary for the installation of gravel. Compaction of the soil reduces the percolation of the effluent and reduces effluent filtration.
Additionally, gravel and pipe leach-field systems require a lot of space and in most cases, an adequate amount of space is not available. Gravel and pipe systems also require a large amount of gravel that must be hauled in by a dump truck. The time and labor-consuming installation of a pipe and gravel system, along with space limitations, creates higher costs and installation difficulties. Thus, the pipe and gravel drain-field system has grown out of use by septic system installation professionals.
Gravel-less drain-field systems include the use of plastic tubes or pipes perforated with holes throughout and covered with spun-bonded permeable nylon filter fabric. The perforated pipes are typically 20 to 30.5 cm (8 to 12 inches) in diameter and are placed in a 30.5 cm (12 inch) to 61 cm (24 inch) underground trench.
Nitrogen compounds, suspended solids, organic and inorganic materials, and bacteria and viruses must be reduced before effluent is considered purified. These pollutants are reduced or completely removed from the effluent by the soil into which the effluent drains from the leaching chambers. Soil aeration, the presence of oxygen in the soil, and good soil drainage promote nitrification. Nitrification of septic tank effluent is necessary to reduce or eliminate hazardous pollutants and is a biologically sensitive process that is dependent on soil environment. Nitrifying bacteria, found only in the top several feet of soil, converts poisonous organic nitrogen and urea components in effluent into ammonium, nitrites, and nitrates. Nitrates, in turn, are tapped by the amino acids in plant roots and become plant food. Denitrification occurs when oxygen diffusion rates in soil are insufficient to meet the demands from microbial respiration, resulting in sludge. This situation arises when soil is near or at water saturation level and oxygen diffusion is slowest, as is the case when the effluent is essentially covered on top with a solid material, such as the arch-chamber segments, and evaporation and oxygenation is inhibited.